It has been the subject of hot debate in the amenity sector so it was no surprise that this year's Palmstead Nurseries Soft Landscaping Workshop: "Native or Non-native? Which is Best?" was packed.
Flying the flag for native is BREEAM, the long-established and most widely used environmental assessment method for buildings. Land use and ecology make up 10 per cent of the rating for any new project.
The intentions, to attract wildlife and contribute to biodiversity, may be good but resentment has been brewing among landscape architects and other professionals over the restrictions that it imposes on their work.
Planners and designers have been locking horns on this issue for years. When James Hitchborough, Nigel Dunnett and Sarah Price were asked to design the Olympic Park gardens as part of, as organisers promised, "the most sustainable Olympics ever", the horticulturists wanted plants from across the world to create an international garden and wild flower meadows.
Planning consultants including the Environment Agency and the Wildlife Trusts wanted native plants that according to Dunnett, one of nine speakers at the workshop, would have left the park with a subdued palette and species that would not have been in flower during the Olympic Games.
It is a familiar scenario as BREEAM, developed by the Building Research Establishment (BRE), is increasingly made mandatory by planning authorities and clients. Native plants are lauded and non-natives are demonised.
Dunnett believes that BREEAM's focus on native plants stifles creativity and can lead to less biodiversity rather than more.
He is not alone. Landscape Institute members flocked to answer a recent technical committee survey - double the normal response rate. Of those, 42 per cent thought it harms landscape quality while 25 per cent thought it has a positive effect.
Only 28 per cent thought it encouraged landscape innovation, while 35 per cent deemed it discouraging. Asked if it increased landscape sustainability, 37 per cent agreed and 13 per cent said it reduced sustainability.
Somewhere in the middle of these two camps are entomologists. Butterflies have recently been found laying eggs on the everlasting pea, a garden escapee considered a pest by conservationists, while studies have shown that insects are not as fussy as BREEAM assessors.
In 2010, the RHS Plants for Bugs study set out to discover whether bugs care about plants' geographical origins. The team, led by senior horticultural adviser Helen Bostock, has studied thousands of spiders, ground beetles, springtails, millipedes, beetles, bees, butterflies, wasps and flies on nine plots planted with native, near-native and exotic plants.
Senior entomologist Dr Andy Salisbury gave a sneak preview of early results. Based on numbers alone, the team discovered that bumblebees, honeybees and omnivore and Miridae bugs prefer the near-native mix, while mites, aphids, springtails and most true bugs prefer native. Wasps went for the exotic mix but solitary bee results were too close to call.
"It may be clear that native is not always best but we have got a long way to go before we can say this is happening," he says. The team plans to publish next year, when a fuller picture will emerge.
BREEAM representative Sarah McCarrick says: "BRE is open to non-native species being used if an ecologist specifies them and could prove a benefit to wildlife."
But even with the help of an ecologist, some BREEAM assessors reject non-natives. Dunnett outlines a project that he would have had to significantly downgrade to comply with BREEAM. After "tens of thousands of pounds worth of effort" to fit a square design in to a round hole, the client gave up on BREEAM altogether.
"The problem is with the interpretation of BREEAM," says Dunnett. "Hundreds of people can be stopped by one planning officer and an ecologically-driven dogma that completely ignores people."
He argues that a "designed nature" approach can be better in urban areas. The Olympic Park's urban meadows were the largest ever created, providing pollinators with a feast of nectar that was not there before. "As with everything, things aren't black and white," he warns.
Environmentalist and wildlife gardener Professor Chris Baines calls for a more sophisticated approach. He told the workshop that a native tree propagated in another country and brought here is not the same as one grown here.
Kew Gardens arboretum head Tony Kirkham wonders why species such as Ginkgo biloba are considered non-native when he has found fossils of them on Scarborough beach. He says climate change may make it impractical to grow familiar species in the future, at least in the south of England.
At the same time, Kirkham acknowledges that Chalara is "a huge threat" and predicts that if pine processionary moths do enter the country they will come in on Italian Pinus pinea.
Baines notes the positive possibilities of development. If it goes ahead, HS2 will carve up the countryside but will also allow horticulturists to restore natural biodiversity alongside the line, replacing less diverse agricultural land.
One thing is clear - there is no shortage of opinions in the native debate. As McCarrick says: "We have a lot of feedback to take on board". BRE plans to publish revised rules very soon.
Key definitions - Native, naturalised and non-native plant varieties
Originated or arrived here of its own accord without human intervention before or as the UK became an island.
Introduced before 1500AD and has become self sustaining and established in the UK.
Generally introduced to the UK by humans. Established and self sufficient. Some are invasive.